1. Field of the Invention
The present invention relates to the field of integrated circuit packaging, and more specifically to a glazed ceramic package with low lead to lead capacitance for integrated circuits usable at high frequencies.
2. Related Art
Almost all semiconductor components are enclosed in packages when they are incorporated into a working electronic system. Typically, integrated circuits are enclosed in an appropriate package prior to their sale by the integrated circuit manufacturer.
There are many reasons for providing a proper encapsulation environment around an integrated circuit chip. The order of importance of these packaging factors depends on the particular component considered, and how it is to be tested and utilized. However, there are several functions of packaging for such chips which are common to all chips and environments. For example, appropriate packaging of an integrated circuit chip generally results in a dramatic economy in assembly. Integrated circuit chips are extremely small and fragile, and therefore are difficult to handle without the aid of magnifying and manipulating equipment. Placement of a chip into an integrated circuit ("IC") package overcomes the difficulty of handling the chip by providing a relatively large and easily handled package configuration. A second important function fulfilled by IC packages is mechanical protection of the chips. Because integrated circuit chips are generally made from fragile materials, they must be protected from the possibility of scratches and chipping. A third essential function of an IC package is to shield the enclosed integrated circuit chip from the surrounding environment so as to preclude corrosion and other detrimental effects resulting from exposure to the normal environment.
A number of different IC package designs are available. Some IC packages are constructed of multilayers of high purity alumina ceramic interconnected with refractory metal pads. These IC packages, known as Flatpaks, include brazed-on lead frames for electrically connecting a chip. Unfortunately, Flatpaks are relatively expensive to manufacture. A less costly IC package design is embodied in "ceramic-glass IC packages".
Ceramic-glass IC packages generally comprise two basic components: (1) a base having an embedded lead frame, and (2) a cap. In the production of these ceramic packages, glass material is screened onto a ceramic base (or substrate) consisting typically of a single pressed and/or fired layer of ceramic. The glass material is distributed so as to cover the base, leaving uncovered at least a small chip cavity which is generally in the center of the substrate. The small chip cavity ("die attach cavity") is typically metallized to provide an attachment site ("die attach pad") for an integrated circuit chip before the glass is screened onto the base substrate. Next, the metallic lead frame is positioned in the glass material on the package base such that when the base is heated to an elevated temperature, the glass material becomes molten and flows around the lead frame. After the glass material has cooled, the lead frame is embedded in the glass material, spaced apart from the package base.
The package cap is manufactured in essentially the same way as the package base, but without the embedding of a lead frame. In other words, the ceramic material which will become the package cap is screened with a glass material so as to cover all but a central area in the cap. The cap and glass material are then fired in a furnace to allow the glass material to adhere to the package cap material.
One frequently utilized ceramic-glass IC package is the ceramic dual in-line package ("CERDIP"). In a CERDIP the body of the package is constructed of an extremely hard ceramic material, and the lead frame consists of metal leads configured in two parallel rows which extend from the body in line with one another.
A second frequently utilized ceramic-glass IC package is the "CERQUAD". As in the CERDIP, the body of the CERQUAD is constructed of a hard ceramic material. However, the lead frame consists of metal leads configured as two sets of two parallel rows perpendicular to one another, typically with the ceramic body in a square configuration.
An integrated circuit manufacturer will use a package base and package cap to form a CERDIP or a CERQUAD in the following manner: an integrated circuit chip is placed in the central chip cavity in the package base; the integrated circuit chip is affixed to this central cavity by bonding it to the metallized layer, and is electrically connected, via minute wires, to the portion of the lead frame which extends around the central cavity; once the integrated circuit chip is electrically and mechanically affixed to the package base, the cap is placed on the base and the entire unit is fired at a temperature higher than the temperature used to adhere the glass material to the package base and cap. During this last firing, the glass material may undergo a devitrifying process so as to render the glass material similar to a crystalline ceramic. This crystalline glass, having a greatly increased mechanical strength, then securely couples and seals the cap to the base, resulting in a single device. In most cases, however, a vitreous, non-devitrifying glass is used, which has mechanical strength equivalent to the devitrifying glass prior to devitrification.
A great deal of research has been performed to develop glass materials which can be accurately screened onto ceramic materials forming ceramic-glass IC packages, fired at a relatively low temperature to permit affixing of the glass material to the ceramic material, and then fired at higher temperatures to make the final seal.
The development of a relatively low firing temperature solder glass has greatly contributed to the higher reliability of certain integrated circuits. Unfortunately, this increase in reliability was accompanied by a corresponding and substantial detriment. Specifically, to alter the chemical composition of the glass material, it was necessary to introduce materials such as zirconium and titanium. The introduction of such materials into the glass, however, has significantly increased the dielectric constant in the glass, resulting in a significant increase in the parasitic capacitance between the closely aligned leads of the lead frame. This high parasitic lead capacitance has, in certain situations, precluded the utilization of low temperature ceramic-glass IC packages for certain integrated circuit chips. Also, even if some integrated circuit chips can be successfully utilized in a low temperature ceramic-glass IC package at a relatively low frequency, they cannot be used at higher frequencies because excessive parasitic capacitance results in undesirable attenuation, distortion, and cross-talk of the electrical signals transmitted by the leads of the package.
It would be very desirable to have a low temperature ceramic-glass IC package which may be used for high frequency applications and which may be produced at relatively low cost. The present invention provides such a package.